1. Field of the Invention
The present invention concerns an apparatus for operating a converter-fed asynchronous machine by a flux computer.
If the magnetic field in the asynchronous machine is described by a flux vector extending in the direction of the field axis, and if the currents flowing in the stator windings are assembled to a current vector, it is advantageous for converter-fed asynchronous machines to control the stator currents so that the flux is kept, e.g. at a constant value with the component of the stator current parallel to the flux vector (magnetizing current), and that a desired value of the torque or of the speed can be set with the component perpendicular to the flux vector (active current). Such a field-oriented operation is known from U.S. Pat. No. 3,824,437. Desired values are preset which fix the stator current vector in a ("field-oriented") coordinate system rotating with the flux vector in amount and angle, or in the Cartesian components. In order to obtain therefrom corresponding control quantities for the stator current to be supplied by the converter, the preset field-oriented nominal stator current vector must be transformed into the stationary reference system of the stator windings, which requires data about the position of the flux vector. The information can be obtained by a flux computer from the corresponding values of stator current and stator voltage. According to U.S. Pat. No. 3,593,083, this can be done by subtracting the ohmic voltage drop on the respective stator winding, which is given by the product of the current through the stator winding and the ohmic resistance, and the inductive voltage drop, which is given by the product of the respective stray inductance and the differentation of the line voltage with respect to time, from the star voltage of the machine terminal. On the winding, the emf appears from which the flux induced in the direction of the respective winding, is computed by integration. Just as the stator current, the stator voltages, the ohmic voltage drops, the stray voltages, the emf and the flux are vectorial quantities which can be assembled from the individual quantities belonging to the individual stator windings to a common vector. In a three-phase induction machine it is thus possible to determine two components of the emf vector or of the flux vector in a coordinate system formed of two winding axes enclosing an angle of 120 deg. by determining the emf or the flux. By means of a coordination transformer it is possible to change from this oblique-angled coordinate system to a corresponding stationary Cartesian coordinate system, which is designated with subscripts .alpha., .beta., in contrast to the field-oriented coodinates designated with subscripts .rho.1, .rho.2. The transformation can be effected at the output of the flux computer, but it can already be done during the determination of stator voltage, so that the stator current vector and the stator voltage vector in the oblique-angled or Cartesian coordinate system can be preset in the flux computer. It is also possible to effect the integration leading from the emf vector to the flux vector after the ohmic voltage drop has been subtracted from the stator voltage vector, and to subtract the product of the stray voltage and the stator current after the integration in order to take into account the inductive stray voltage. Such a flux computer is known, for example from German Patent No. 2,833,542 corresponding to U.S. Application No. 58,830 "Rotating-Field Machine Drive and Method".
This flux computer, which is based substantially on the stator voltage vector and forms the flux vector by integration of the emf, can be called a "voltage model". This model frequently meets the demands for accuracy and control dynamics in the operation of an asynchronous machine even without the exact knowledge of the stator resistance, provided the stator frequency of the asynchronous machine exceeds the rated frequency by about 10%. The voltage drop on the stator resistance is then negligible relative to the emf, so that an inaccurate determination and setting of the ohmic voltage drop has little effect on the flux determination. For lower frequencies, however, an exact determination of the flux depends on the accurate knowledge of the temperature-dependent stator resistance R.sup.S. Furthermore, the open integration method used in the voltage model requires an elaborate, adaptive, DC-components control to avoid zero point drift, which leads at low frequencies to falsification of the flux determination, and also impairs the control dynamics in the upper frequency range.